1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. More particularly, it relates to a highly sensitive and durable electrophotographic photoreceptor.
2. Prior Art
In these days, an electrophotographic technique, which may instantly produce an image with a high quality, has been widely used and applied in the fields of various kinds of printers as well as of a copying machine.
As photoconductive materials for the photoreceptor which is one of the essential part of the electrophotographic technique, inorganic ones such as selenium, arsenic-selenium alloy, cadmium sulfide and zinc oxide have been generally used. In addition, organic photoconductive materials have been recently used for the photoreceptor because they have many advantages over the inorganic photoconductive materials, for example, they are light in weight and may be easily prepared and formed into a film.
As the organic photoreceptor, there have been known of a so-called dispersed type in which fine photoconductive powder is dispersed in a binder resin and of a layered type comprising a charge generating layer and a charge transporting layer on an electroconductive support. Please refer to, for example, U.S. Pat No. 4,396,696. The latter type is mainly put to a practical use in view of its high sensitivity and high durability against printing.
However, the sensitivity and durability of the conventional organic layered-type photoreceptor are still insufficient as compared with inorganic one which uses arsenic-selenium alloy. Therefore, various attempts have been made for further improving such properties.
New photosensitive material with higher sensitivity has been sought for improving the sensitivity of the photoreceptor, while photosensitive material which will deteriorate little and binder material with high mechanical strength have been also sought for improving its durability. As a result, materials having a sufficient sensitivity and electric durability have been successfully developed. However, the photosensitive material with a sufficient mechanical durability has been not yet obtained.
Consequently, a photosensitive layer may be abrased and its film thickness may accordingly be decreased by a practical load such as friction with toner or paper, or friction with a cleaning member although a degree of the decrease depends on the method and load used. Such decrease in the film thickness may result in reduction of a charging property and, when the reduction exceeds an allowable range in a developing system, the life of the photoreceptor will expire so as to deteriorate the durability against printing.
The mechanical durability may vary mainly depending on the binder resin for the charge transporting layer. Although acrylic resin, methacrylic resin, polyester resin, polycarbonate resin and the like are usually used for the binder resin, these materials have not yet been provided with a sufficient strength in the prior art. Accordingly, when they are used in a process having a normal blade-cleaning system, the photosensitive layer will be remarkably abrased by copying for several tens of thousands of sheets, causing the need of replacement thereof. Although varying depending on the resin material and process, the decrease of the film in thickness caused by such abrasion is usually about from 0.2 to 1 .mu.m after copying ten thousands of sheets. Various studies have been therefore made on the conditions of use and on new materials in order to decrease an amount of said abrasion.
The present inventors have made various studies to find a method of improving the durability while using various conventional materials, and have found that the change of electrical properties due to the abrasion, particularly, the reduction in a charging capacity can be prevented by sufficiently increasing the film thickness of the photosensitive layer as compared with the conventional ones, specifically, by greatly increasing the film thickness of the charge transporting layer.
However, for an usual layered-type photoreceptor, the electrical properties were proved to be remarkably degraded by increasing the film thickness of the charge transporting layer, causing a decrease in the sensitivity and a remarkable increase in a residual potential, which can be no more suitable to practical use.
It has been now found, however, that the above disadvantages may be compensated, or rather the sensitivity may be improved as long as the layered-type photoreceptor has specific electric properties, even if the thickness of the charge transporting layer is made much thicker than the conventional layer of about 10 to 20 .mu.m thickness. Consequently, photosensitive material with more excellent durability and higher sensitivity as compared to the conventional one may be obtained.